The present invention relates to a flow trim for use in a choke valve for preventing or reducing fracturing of flow trim choke components.
A choke valve is provided on the production wellhead to provide controlled production of an oil or gas reserve. By controlling the orifice through the valve, a pressure drop and flow rate is controlled to ensure the well is produced in a manner to maximize production output, meet production quotas, minimize sand or solid production and ensure the formation is not damaged through the process. The manner in which production reserve is produced and managed can significantly affect the quantity of the reserve.
A problem that has produced many production interruptions both with surface and subsea facilities is the failure of valve trim due to fracture. The common causes of this fracture mechanism are foreign debris moving through the flow line into the valve with sufficient mass and velocity to damage or fracture the valve trim or the effect of liquid slugs in a high velocity gas phase entering the valve inlet. Choke valve trim is typically manufactured from tungsten carbide material. This material is a powder metallurgy product where tungsten in the concentration of about 85-95% is bonded within a matrix with a binder material, typically nickel, cobalt, molybdenum, chromium or a combination of these elements in the concentration of about 5-15%. This produces a material that is very hard by nature to prevent or delay the effects of erosion to the valve trim. The hardness of the trim components is typically in the Rockwell Ra 90-95 range. A negative trait of the tungsten carbide material matrix is the susceptibility to fracture due to impact damage.
There have been a number of attempts at solutions to mitigate the damage caused by foreign debris; however, many of these attempts have resulted in a trade off to the potential wear capabilities of the choke trim. Two such solutions are described below.
1. Some choke valve manufacturers have adopted a standard to select tungsten carbide Grades that have a higher percentage of binder material typically nickel, cobalt, molybdenum, chromium or a combination of these elements. Concentrations in the order of 12-15% have resulted in improved toughness of the tungsten carbide matrix; however, the volume of binder in the concentration results in a matrix with lower hardness and consequently substandard erosion characteristics. This results in rapid wear to the flow trim resulting in costly production interruptions for valve maintenance or in the subsea applications, a high valve retrieval cost to facilitate maintenance.
2. Another option used by some choke valve manufacturers is to encase the tungsten carbide material with a stainless steel carrier, thereby absorbing some of the energy from the foreign particle on impact and protecting the brittle tungsten carbide from direct impact. A negative consequence of this commonly known “plug and cage trim” design is that in order to encase the tungsten carbide in a stainless steel carrier, a valve design must incorporate an internal plug, in a design commonly referred to an internal plug cage style trim. In this way, the tungsten plug material travels inside the cage protected from impact. The tungsten carbide material required to resist wear to the cage is protected by a stainless steel carrier. This internal plug design has proven inferior for erosion resistance when compared to the external sleeve (flow collar) choke valve designs.
External sleeve flow trim designs for choke valves are generally described in the prior art, see for instance U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove et al., and U.S. Pat. No. 6,105,614, issued Aug. 22, 2000 to Bohaychuk et al. A choke valve including an external sleeve flow trim in subsea applications is shown in U.S. Pat. No. 6,782,949 to Cove et al. (see FIG. 1 prior art). These patents describe the beneficial characteristics of the external sleeve (flow collar) design in erosion control, valve outlet erosion protection, seating integrity, and fluid energy control features.